Purification of naphthenic acids



Patented Feb. 15, 1938 PUREICATION OF NAPHTHENIC ACIDS John T. Rutherford, Berkeley, Calif., assignor to Standard Oil Company of California, San Francisco, Califl, a corporation of Delaware No Drawing. Application July 3, 1933, Serial No. 678,996

3 Claims. (oi 260-108 This invention relates to the preparation of naphthenic acids from petroleum oils and to the purification of the same for use in the arts and industries. ,In particular, it refers to the prepara- 5 tion of naphthenic acids which do not contain appreciable quantities of unsaponiflable matter of molecular weight or boiling point range similar to those of the separated and purified acids, which are practically colorles, and which possess none of the strong sour odors commonly associated with naphthenic acids as heretofore prepared.

As used herein, the term naphthenic acids is intended to apply to the naturally occurring carboxylic acids of petrdleumoiis, ordinarily removable from crude petroleum oils or their distillates by'treating with a dilute aqueous alkaline solution; it is not intended that the term shall apply to the so-called petroleum sulfonic acids which are formed when petroleum oils or their distillates are treated with astrong sulfuric acid or with sulfuric 'anhydride.

Ordinarily, naphthenic acids are obtained by washing a petroleum oil or its distillate with a dilute caustic alkali solution; the alkali metal salts of the acids are formed and are removed in solution in the aqueous layer. Alternatively, the petroleum oils or their distillates are neutralized with an alkali and the neutralized oil subjected to distillation; the alkali metal naphthenates remain as a residue in the still bottoms fromsuch a distillation. In either of these two processes the naphthenicacids, as recovered byany ordinary means (generally by acidification of the alkali metal salts or soaps with a dilute mineral acid), are highly contaminated with neutral hydrocarbon oils, color bodies and malodorous substances which require removal prior to. the use of the acids or their salts for any of the purposes of the arts and industries. For example, the presence of neutral unsaponifiable oil is objectionable in a naphthenate siccative because it adds to the paint or varnish a heavy, tacky, non-drying oil.

naphthenic acid esters when used. as nitrocel iulose plasticizers and the like. The technical difliculty of removing from naphthenic acids or their salts the unsaponifiable matter, color bodies and malodorous substances and malodorous substances is objectionable in acids islabout 31.5% by tions of either increasing the solubility of the neutral oil in the aqueous medium. Even with very dilute naphthenic soap solutions'and in the absence of any excess alkali, I have found that an amount of neutral hydrocarbon oil equal to as much as by weight of the naphthenic acids themselves, may be dissolved.

Ordinarily, a simple test for percent unsaponifiable matter would be suilicient to ascertain the purity of 'a substance of thischaracter. Since alkali metal naphthenates have the property of taking neutral oils into solution, however, such a test is not satisfactory as a criterion of purity as regards naphthenic acids. The purity or freedom'of naphthenic acids from neutral oils of the same general molecular weight or boiling point range may be taken as in direct proportion to acid number. For example, purified naphthenic acids from California petroleum lamp oil distillates' have an average acid number of about 250. The crude naphthenic acids prepared by the ordinary extraction of such a distillate with a dilute aqueous alkaline solution have an acid number of about 190. From this it appears that the amount of neutral oilin the crude naphthenic weight-calcu1ated as follows: l

For convenience, acid numbers are referred to herein as a criterion of purity of naphthenic acids.

It is an .object of my invention to prepare naphthenic acids from petroleum oils which are free from neutral hydrocarbons, are extremely light colored or colorless, and possess relatively mild and unobjectionable odor.

It is a further object of my invention to disclose and provide naphthenic acids suitable for the preparation of naphthenic acid salts which may be used in the arts and industries for such purposes aspreclude the presence of the uncrude naphthenic acids, substantiallf'complete separation of all unsaponifiable matter from the naphthenic acids may be effected by ordinary distillation processes. In the process, .a petroleum gas oil or similar petroleum distillate containing naphthenic acids is treated with a caustic alkali solution, preferably one about 4 as gravity. The naphthenic acids are remo'ved in the aqueous layer as sodium soaps. As noted above, the aqueous layer, upon separation from the 011, contains considerable quantities of neutral hydrocarbon oils in clear solution. 'Ihis aqueous layer is extracted witha light oil solvent, for example, a treated gasoline of relatively high volatility and low end boiling point. By this extraction, a part of the heavy neutral oil contained in solution in the aqueous layer is replaced by the light solvent, an amount of the heavier neutral oil equivalent theretovpasslng to the supernatant layer.

In the process of my present invention, I make use of this principle of replacing the heavy neutral oil originally accompanying the naphthenic acids with a lighter refined neutral oil later ca,-

pable of separation by ordinary distillation means, and I combine this principle with a treatment of the acids with a relatively strong sulfuric acid, thereby obtaining also the benefit of a chemical refining action.

The various steps requisite to the refining and purifying process of my present invention may be carried out as follows:

A petroleum gas oil or similar petroleum distillate containing naphthenic acids is treated with a caustic alkali solution, ordinarily one of about 4 B. strength. The naphthenic acids are removed in the aqueous layer as alkali metal soaps. As described above, the aqueous layer, after its separation from .the petroleum oil, contains considerable quantities of neutral hydro carbon oils in clear solution, as well as color bodies and malodorous substances. This aqueous naphthenate solution, after settling clear of entrained oil, is acidified with a-dilute mineral acid and the crude naphthenic acids, accompanied by their impurities, are skimmed 03. These crude acids are very dark colored, contain large quantitles of neutral unsaponifiable matter, and possess the strong sour odor characteristic of the naphthenic acids of commerce. 7

To the crude naphthenic acids is now added a light solvent, water-immiscible, and preferably of a boiling point range-considerably lower than that of the heavy neutral oil, such, for example, as a refined light gasoline of relatively low end boiling point. The solution of crude naphthenic. acids in the light solvent is then' treated with a small quantity of relatively strong sulfuric acid, sufllcient to react with the heavy neutral oil originally accompanying the crude acids upon their extraction fromthe petroleum oil, and sufficlent to react upon or combinerwlth the color bodies and malodorous compounds present as im-f purities. Iprefertotreatthenaphthmicacids ordinary distillation means.

' and retreated with sulfuric acid, as before.

with sulfuric acid in the presence of the solvent because of the viscosity of the naphthenic acids themselves; thus, when undiluted with solvent or a diluent, it is practically impossible to secure adequate contact of the acids with the treating agent; moreover, the viscosity of the acids, when undiluted, prevents separation of the sulfuric acid sludge after the treatment. When diluted, prefluted naphthenic acids with a dilute aqueous caustic alkali solution. The treated naphthenic acids pass into solution in' the aqueous caustic as .alkali metal soaps, the light gasoline remaining as an immiscible layer above. As noted hereinabove, the alkali metal salts of these acids have the power of carrying a considerable amount of neutral oil into clear solution in an aqueous medium; it will be apparent. that here, where the aqueous solution is in contact with a light solvent of diflerent boiling point range from that of the neutral hydrocarbons originally accompanying the acids, the specific. amount of neutral oils which are carried into aqueous solution is made up of light, treated solvent, of characteristics such that it may later be removed by simple and After separation of the clear aqueous alkaline naphthenate layer from the immiscible solvent layer, the purifiednaphthenic acids are recovered or freed by acidification with a dilute mineral acid. Such amounts of neutral oil as accompany the acids, after acidification and separation of the acid andaqueous'layers, may be removed by ordinary volatilization, preferably. at reduced pressure in order to avoid decomposition of the acids.

Although not often necessary, these purified acids may again be diluted with a light solvent Ordinarlly this need not be done, for the acids provided by a single treatment with sulfuric acid, as described above, are very light in color, contain little or no neutral unsaponifiable matter, have very high acid numbers, and contain little or no malodorous impurities.

As an example of the purification obtained in Y the process of my presefitinvention:

' strong sulfuric acid sludge, the naphthenic acids in solution in the light gasoline were s'aponified with 8 B. caustic soda solution. The clear sodium naphthenate solution was removed from the supernatant solvent layer and the purified acids were liberated by acidification with diluted sulfuric' acid. These "purified acids were skimmed off and such light solvent as accompanied them was removed by volatilizatlon under reduced pressure. The naphthenic acids had a color of 4 N. P. A., and acid number of 250, and a mild unobjectlonable odor.

Metal naphthenates and naphthenic esters prepared from these acids were free from neutral oils, were light colored and odorless, satisfactory for use under the most exacting requirements for siccatives, nitrocellulose plasticizers, and the like.

The amount of the sulfuric acid used to purify the napthenic acids, as well as the strength of the acid, may be varied considerably. For example, as small a quantity of 66 B. acid as 0.5 pound per gallon of crude acids will effect marked improvement in their acid number, color and odor. Again, I have used as much as 2.0 pounds per gallon of crude naphthenic acids, and in such a case I obtain a purified product which is almost colorless, and of a bland, practically odorless nature even when heated. Some loss in yield of naphthenic acids is suffered upon treatment with large quantities of sulfuric acid,and .I am guided therefore in the severity of my treatment by the use to which the product is to be put, and by the requirements to be met in such use.--In general, sulfuric acid of below about 80-85% H2804 is of but little benefit in my process of purification. I have found that 90-98% H2504 most'satisfactorily accomplishes the removal of unsaponiflable matter, color bodies, odorous substances and the like impurities, and I therefore prefer to use an acid within. this range of strengths for the purposes of my invention. i Y

While I havedescribed in detail the character of my invention andgiven illustrative examples tation should be imposed upon the invention. thereby.

I claim: 1. The process of purifying naphthenic acids which comprises treating crude naphthenic acids wth a strong sulfuric acid in the presence of a 2. The process'of purifying naphthenic acids which comprises treating crude naphthenic acids with a strong sulfuric acid in the presence of a water-immiscible diluent more-volatile than the naphthenic acids and relatively inert to sulfuric acid treatment, separating the sulfuric acid sludge, saponifying the treated acids with an aqueous alkali in the presence of the'diluent, separating the diluent phase, acidifying the aqueous naphthenate solution with a mineral acid, and removing the volatile diluent accompanying the purified acids by volatilization.

3. A process of preparing naphthenic acids from petroleum oils which comprises extracting a petroleum oil with a dilute aqueous alkaline solution, acidifying the aqueous naphthenate solution with a dilute mineral acid, separating crude naphthenic acids, diluting the acids with about four times their volume of a refined hydrocarbon distillate of end boiling point not over about 325 F., treating the naphthenic acids in the presence of the hydrocarbon distillate with between 0.5 and 2.0 pounds of strong sulfuric acid per gallon of naphthenic acids, the said strong sulfuric acid having a strength greater than 80% H2804, removing sulfuric acid sludge, saponifyingthe treated acids with an aqueous alkali in the presence of the hydrocarbon distillate, separating the resulting aqueous naphthenate solutionfrom the supernatant hydrocarbon distillate layer, and acidifying the aqueous naphthenate solution with a dilute mineral acid.

' JOHN T. RUTHERFORD. 

